Current Biology
Volume 27, Issue 12, 19 June 2017, Pages 1776-1781.e9
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Infrasonic and Ultrasonic Hearing Evolved after the Emergence of Modern Whales

https://doi.org/10.1016/j.cub.2017.04.061Get rights and content
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Highlights

  • The cochlea, headquarters of hearing, is described for two ancient protocetid whales

  • These amphibious whales had hearing capacities close to those of their terrestrial kin

  • Extreme hearing abilities in whales derive from a mid-frequency ancestral ear

Summary

Mysticeti (baleen whales) and Odontoceti (toothed whales) today greatly differ in their hearing abilities: Mysticeti are presumed to be sensitive to infrasonic noises [1, 2, 3], whereas Odontoceti are sensitive to ultrasonic sounds [4, 5, 6]. Two competing hypotheses exist regarding the attainment of hearing abilities in modern whales: ancestral low-frequency sensitivity [7, 8, 9, 10, 11, 12, 13] or ancestral high-frequency sensitivity [14, 15]. The significance of these evolutionary scenarios is limited by the undersampling of both early-diverging cetaceans (archaeocetes) and terrestrial hoofed relatives of cetaceans (non-cetacean artiodactyls). Here, we document for the first time the bony labyrinth, the hollow cavity housing the hearing organ, of two species of protocetid whales from Lutetian deposits (ca. 46–43 Ma) of Kpogamé, Togo. These archaeocete cetaceans, which are transitional between terrestrial and aquatic forms, prove to be a key for determining the hearing abilities of early whales. We propose a new evolutionary picture for the early stages of this history, based on qualitative and quantitative studies of the cochlear morphology of an unparalleled sample of extant and extinct land artiodactyls and cetaceans. Contrary to the hypothesis that archaeocetes have been more sensitive to high-frequency sounds than their terrestrial ancestors [15], we demonstrate that early cetaceans presented a cochlear functional pattern close to that of their terrestrial relatives, and that specialization for infrasonic or ultrasonic hearing in Mysticeti or Odontoceti, respectively, instead only occurred in fully aquatic whales, after the emergence of Neoceti (Mysticeti+Odontoceti).

Keywords

bony labyrinth
cochlea
Protocetidae
archaeocetes
Artiodactyla
Cetacea
Eocene
fossil
CT scan

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